Prove of the concept: $50 400mm set up

Last week, I believe, I annoyed a few people here asking kind of stupid questions about manual long zooms. There was a Vivitar 400mm 1/5.6 lens for $175 on eBay (may be it's still there) that I was thinking of. However last Sunday I was lucky to get another lens at local camera show. And there is my new set up: K-x + Super-Takumar 1:4/200mm M42 + Helios 2x M42 converter + Pentax K-mount adapter. it is compact and is not really heavy.

Quick and dirty tests show that this combination is at least as sharp or even better than my DAL 55-300mm at 300mm, cropped to the same size as 400mm shot.

In the same time I haven't had a chance to test it at bright daylight yet. The Takumar combo image looks a bit noisier and I'm not sure what is a cause: the lens, cheap teleconverter, or just a dim light (I tested it at ISO 400).

Which M42 2x or 2x+ converter is better to acquire (brighter, better quality, etc.)? And what is a better technique to focus manually such a long zooms? I can't keep it steady enough :-(. Would a bigger massive ball head help?

BTW: I also purchased a very nice smc Pentax-M 1:1.7 50mm K-mount piece and met a few Pentaxians over there.
Thank you in advance
SG